At least five people were killed and several others injured on Thursday (Jun 28) when a gunman opened fire inside the offices of the Capital Gazette, a newspaper based in Annapolis, a historic city an hour east of Washington.
Police described the suspect - who was being questioned in custody - as a white adult male in his "late thirties" who was armed with a rifle or a shotgun, and appeared to have acted alone.
Law enforcement in Baltimore and New York City deployed protective forces to major media outlets as a precaution, authorities said.
"The shooter is in custody and is being interrogated at this time," Anne Arundel County Executive Steve Schuh told a news conference.
Maryland Governor Larry Hogan said at the news conference: "It's a tragic situation. We don't have all the information yet, and we can't give all the information yet, but we have had several fatalities."
This was a targeted attack on the Capital Gazette," Bill Krampf, acting police chief of Anne Arundel county, told reporters during a briefing.
He said that police did not yet know the shooter's motive but: "We know that there were threats sent to the Gazette through social media."
"We're trying to confirm what account that was and we're trying to confirm who actually sent them," Krampf said.
Krampf said that the threats did not appear to target an individual but rather the media outlet as a whole, adding "they indicated violence."
Krampf added the attacker had used canisters of smoke grenades while entering the office building and "was prepared today to come in."
For now, the Annapolis shooting is being treated as a local incident and not one that involves terrorism, a law enforcement official said. The Federal Bureau of Investigation is on the scene assisting local authorities, the official said.
A reporter for the Capital Gazette, Phil Davis, tweeted that a "gunman shot through the glass door to the office and opened fire on multiple employees." He said several people were killed.
"There is nothing more terrifying than hearing multiple people get shot while you're under your desk and then hear the gunman reload," Davis said.
Up to four people had been killed, according to CBS News quoting two sources.
The newspaper is located in a four-storey office building in Annapolis, the capital of the US state of Maryland.
President Donald Trump has been briefed on the shooting, White House spokeswoman Lindsay Walters said.
"Our thoughts and prayers are with all that are affected," she said. Trump was aboard Air Force One, returning to Washington from an event in Wisconsin.
One law enforcement source told CBS News the suspect is a male in his 20s who had no identification on him. Two law enforcement sources told CBS News the suspect used a shotgun.
Police also went to the offices of the Baltimore Sun as a precaution, that paper reported.
The New York Police Department said it was beefing up security at New York-based news organisations as a precaution.
"We're deploying units from our Critical Response Command to news outlets throughout New York City," said Officer Andrew Lava, an NYPD spokesman.
"There is no active threat at this time," he said.
Agents from the Baltimore office of the Bureau of Alcohol, Tobacco, Firearms and Explosives were responding to the incident, the bureau tweeted.
Police are checking the building in Annapolis for explosives and whether more than one suspect was involved, Anne Arundel County police spokesman, Lieutenant Ryan Frashure, told reporters.
Live video images showed people leaving the building, walking through a parking lot with their hands in the air. Scores of police vehicles were at the scene.